In high-speed optical communication systems digital information is usually encoded by intensity modulating an optical signal as illustrated in FIG. 1 and described in the following. There is a continuous stream of bits at a rate B into the system. At the transmitter, a binary “1” digit corresponds to a high optical power, whereas a binary “0” digit corresponds to a relatively low optical power. The signal is then transmitted over an optical fiber and at the receiving end a detector converts the optical input signal to an electrical signal. The electrical signal is then fed to a decision circuit, which samples the signal at specific times and compares the sampled signal to a threshold value. If the sampled value is above or below the threshold the receiver outputs a digital “1” or “0” symbol respectively. Under ideal conditions, the output symbols exactly match the input symbols.
Practically, a signal will for various reasons be degraded when it travels through the different components of a transmission system and then there is a problem of finding or setting an appropriate threshold in binary detection. An appropriate threshold value will give the best possible detection for a given input signal. Many documents deal with this problem of setting an appropriate threshold.
Thus, in U.S. Pat. No. 4,823,260, a binary data regenerator is disclosed which uses three threshold levels, one level located between the other two, the first two levels produced by two control loops such that predetermined error rates occur on the ZERO and on the ONE, conveniently the same error rate for both. The bit error rate (BER) is established separately for the ZERO and the ONE.
In U.S. Pat. No. 5,806,641, a decision threshold setting circuit for e.g. an optical receiver is disclosed in which the average input current is used in order to achieve a variable threshold.
In U.S. Pat. No. 4,456,992, enhancement of the probability of error-free reception of digital data is described, by means of a time-variable threshold value.
In U.S. Pat. No. 5,896,422, a method for adaptively setting of the decision threshold in digital communication is disclosed, in which the average bit amplitude is used.
In U.S. Pat. No. 5,923,219, an automatic threshold control circuit is disclosed, that detects the minimum level of an input signal, and a maximum level relative to the minimum level, and generates a threshold level from these values.
U.S. Pat. No. 4,847,865, an improved circuit for threshold adjustment in digital communication is disclosed.
In U.S. Pat. No. 5,459,311, a fiber optic system including a threshold producing circuit is described, the decision threshold voltage preferably being half the peak voltage.
In U.S. Pat. No. 4,318,128, reception of digital data in the presence of noise is disclosed. A threshold value is set at an average value of a voltage, in which the noise level is taken into account.
In the published European patent application 0 240 157 an optical communication system is described that includes an automatic threshold value controlling circuit for setting the threshold at an average value of the input intensities.